1. Field of the Invention
The present invention relates to a circuit for driving a piezoelectric transformer performing a voltage-to-voltage conversion by utilizing a piezoelectric element, and more specifically to a piezoelectric transformer driving circuit capable of realizing a high conversion efficiency even if an input voltage varies in a wide range.
2. Description of Related Art
In general, a piezoelectric transformer can be said to be a voltage-to-voltage converter so configured that a mechanical vibration is caused in piezoelectric element by applying a voltage between primary electrodes, and a different voltage is picked up from secondary electrodes by action of a piezoelectric effect of the piezoelectric element. The piezoelectric transformer is featured in that it is easily scaled down and thinned in comparison with an electromagnetic transformer, and is used as an inverter for lighting a cold-cathode fluorescent lamp (one kind of cold-cathode tube) and attracts attentions as a high voltage power source.
The present invention is directed to a driving circuit for this piezoelectric transformer, which is used as an inverter for converting a DC voltage to an AC high voltage or a DC-to-DC converter for converting a DC voltage to a DC high voltage. For example, in the case that the piezoelectric transformer is used in a power source for a back light incorporated in a 9.4-inch color liquid crystal display unit, if a load is a cold-cathode fluorescent lamp (cold-cathode tube) having a tube length of 220 mm and a tube diameter of 3 mm.phi., the inverter is required to convert a DC voltage in the range of 5 V to 20 V, to an AC power of a lamp voltage of about 500 Vrms (the suffix "rms" indicates a root-mean-square value), a lamp current of about 5 mArms and a frequency of 100 kHz. In the case that the piezoelectric transformer is used as a power supply for generating a static electricity in a laser printer, it is necessary to obtain a DC voltage on the order of 1 kV to 5 kV from a DC power supply on the order of 24 V.
In Japanese Patent Application No. Heisei 7-069207 filed on Mar. 28, 1995 and its corresponding U.S. patent application Ser. No. 08/623,028 (the disclosure of which is incorporated by reference in its entirety into this application), the inventor of this application has proposed one piezoelectric transformer driving circuit, a block diagram of which is shown in FIG. 1.
The piezoelectric transformer driving circuit shown in FIG. 1, comprises a piezoelectric transformer 1 having a pair of primary electrodes and a pair of secondary electrodes, a load 2 connected to the secondary electrodes of the piezoelectric transformer 1, a step-up or boosting circuit 4 connected to the primary electrodes of the piezoelectric transformer 1 for driving the piezoelectric transformer 1 with an AC voltage, and a frequency control circuit 3 connected to the load 2 (or the secondary electrodes of the piezoelectric transformer 1) and for controlling the boosting circuit 4 to the effect that the piezoelectric transformer 1 is driven at or near a resonance frequency of a piezoelectric element so that an output is controlled at a constant level. This piezoelectric transformer driving circuit is an inverter receiving a input DC voltage VDD and supplying an AC output voltage VO(Vrms) to the load 2.
With this arrangement, if a driving voltage of a sine wave having the resonance frequency is applied to the primary electrodes of the piezoelectric transformer 1, the piezoelectric transformer causes a mechanical vibration. The piezoelectric transformer 1 has a step-up ratio determined by a shape of the piezoelectric transformer, and therefore, the AC voltage elevated by the step-up ratio can be obtained from the secondary electrodes of the piezoelectric transformer 1.
Referring to FIG. 2, there is shown an equivalent circuit of a part composed of the piezoelectric transformer 1, the boosting circuit 4 and the load 2, in the circuit shown in FIG. 1. The piezoelectric transformer 1 can be considered to be constituted of a resonance circuit of series-connected inductor L, capacitor C and resistor R and an ideal transformer T, and to receive a drive voltage supplied from the primary electrodes, and to output from the secondary electrodes a voltage stepped up with a step-up ratio Av. In the case that the piezoelectric transformer 1 is driven with a frequency other than the resonance frequency, a parasitic vibration occurs in the piezoelectric transformer 1, but only a component of the resonance frequency can be extracted from the secondary electrodes, with the result that the conversion efficiency of the piezoelectric transformer 1 drops because of an energy loss. Therefore, it is important to drive the piezoelectric transformer 1 with a sine wave containing no component other than the resonance frequency. In addition, since it is possible to generate a sine wave having a voltage higher than an input power source voltage, by use of a coil or an electromagnetic transformer, the piezoelectric transformer driving circuit can be advantageously driven with a lower input voltage.
Under the above mentioned circumstance, the boosting circuit 4 shown in FIG. 1 is so configured that a sine wave is generated with a resonance between an inductance of the electromagnetic transformer and a capacitance of the piezoelectric transformer and the piezoelectric transformer itself is driven with the generated sine wave. In brief, the primary electrodes of the piezoelectric transformer 1 are connected to electromagnetic transformers T1 and T2, respectively, in the form of an autotransformer, and a pair of complementary clocks generated by a 2-phase pulse generator 9 are supplied to a pair of transistors Q1 and Q2, respectively, so as to alternately turn on the transistors Q1 and Q2, so that a primary winding of the electromagnetic transformers T1 and T2 are alternately supplied with an electric current from a DC power source VDD, with the result that the primary winding of the electromagnetic transformers T1 and T2 alternately accumulate an electromagnetic energy.
When each of the transistors Q1 and Q2 is turned off, the accumulated electromagnetic energy is discharged to generate a voltage higher than the input DC power source voltage. In the equivalent circuit shown in FIG. 2, it is so designed that each transformer causes a voltage resonance (series resonance) in cooperation with an input equivalent capacitance Cd1 of the primary electrode of the piezoelectric transformer 1, and a half-wave sine wave having a peak voltage of about three times of the input DC power source voltage VDD is generated. Half-wave sine waves Vd1(Vo-p) (the suffix "o-p" indicates a zero-to-peak value) and Vd2(Vo-p) generated in the electromagnetic transformers T1 and T2 are stepped up by the secondary winding of the electromagnetic transformers T1 and T2 to similar half-wave sine waves VS1(Vo-p) and VS2(Vo-p), respectively, which are turn-ratio "N+1" times of Vd1(Vo-p) and Vd2(Vo-p). These half-wave sine waves VS1 (Vo-p) and VS2(Vo-p) are supplied to the primary electrodes of the piezoelectric transformer 1.
These two half-wave sine waves VS1(Vo-p) and VS2(Vo-p), which are opposite to each other in phase, equivalently becomes one full-wave sine wave having an amplitude of VS1+VS2(Vp-p) (the suffix "p-p" indicates a peak-to-peak value), which in turn vibrates the piezoelectric transformer 1, with the result that the secondary electrodes of the piezoelectric transformer 1 generates a stepped-up AC voltage VO(Vrms), which is determined by the shape of the piezoelectric transformer 1.
The AC voltage VO(Vrms) thus generated is applied to the load 2, so that an AC current IO(mArms) (or alternatively, AC voltage VO(Vrms)) is supplied to the frequency control circuit 3. This frequency control circuit 3 is configured to generate a driving frequency for the piezoelectric transformer 1, to be supplied to the 2-phase pulse generator 9, and to continue to sweep the driving frequency until the AC current IO(mArms) (or alternatively, AC voltage VO(Vrms)) outputted from the piezoelectric transformer 1 reaches a predetermined value, and to stop its sweeping when the AC current IO(mArms) (or alternatively, AC voltage VO(Vrms)) becomes the predetermined value.
As shown in FIG. 1, the frequency control circuit 3 comprises a current-to-voltage converter 10 receiving the AC current IO(mArms), a rectifying circuit 11 receiving an output of current-to-voltage converter 10 (or alternatively the AC voltage VO(Vrms) directly from the piezoelectric transformer 1), an comparator 12 for comparing an output of the rectifying circuit 12 with a reference voltage Vref, an integrating circuit 13 receiving an output of the comparator 12, a voltage controlled oscillator (VCO) 15 receiving an output of the integrating circuit 13, and another integrating circuit 14 receiving the output of the integrating circuit 13 and having an output connected to a control input of the integrating circuit 13.
With this arrangement, the AC current IO(mArms) flowing through the load 2 is converted by the current-to-voltage converter 10 into a voltage signal, which is in turn rectified by the rectifying circuit 11 to a DC detection signal. This DC detection signal is compared with the reference voltage Vref by the comparator 12. When the DC detection signal is smaller than the reference voltage Vref, the comparator 12 outputs a high level signal to the integrating circuit 13, and on the other hand, the integrating circuit 13 is configured to continuously drop its output voltage at a constant rate during a period in which the integrating circuit 13 receives the high level signal. The output voltage of the integrating circuit 13 is applied to the VCO 15, which generates a pulse having a frequency in proportion to the received voltage. This pulse is supplied to the 2-phase pulse generator 9 so that the piezoelectric transformer 1 is driven with the frequency of the VCO 15. Therefore, when the DC detection signal is smaller than the reference voltage Vref, the driving frequency continues to drop.
The following is the reason for sweeping the driving frequency from a high frequency side to a low frequency side. It is constructed to use a frequency region higher than the resonance frequency "fr" of the piezoelectric transformer 1. On the other hand, it is also designed that if the detection signal outputted from the rectifying circuit 11 is lower than the reference voltage Vref, the driving frequency drops. The closer the driving frequency becomes to the resonance frequency "fr", the step-up ratio of the piezoelectric transformer 1 increases, and therefore, the AC current IO (mArms) (or alternatively, the AC voltage VO (Vrms)) continues to increase. In this condition, when the voltage applied to the comparator 12 exceeds the reference voltage Vref, the output of the comparator 12 becomes the low level, which in turn causes the integrating circuit 13 to stop its integration operation. Thereafter, the output of the integrating circuit 13 is maintained at a voltage level just before the output of the comparator 12 becomes the low level. Accordingly, the output frequency of the VCO 15 becomes a constant, and therefore, the piezoelectric transformer 1 is driven with a constant driving frequency, so that the AC current IO (mArms) (or alternatively, the AC voltage VO (Vrms)) supplied from the piezoelectric transformer 1 is maintained at a constant level.
When the input DC voltage VDD is smaller than a rated voltage, or during a period until a cold-cathode fluorescent lamp (cold-cathode tube) connected as the load 2 starts to discharge, a predetermined AC current IO (mArms) (or AC voltage VO (Vrms)) cannot be supplied to the load 2. In this situation, the driving frequency of the VCO 15 drops lower than the resonance frequency of the piezoelectric transformer. Thereafter, when the input DC voltage VDD reaches the rated voltage or more, or when the cold-cathode fluorescent lamp starts to actually discharge, the step-up ratio of the piezoelectric transformer 1 is not sufficient, so that a necessary electric power cannot be supplied to the load 2. Accordingly, when the driving frequency dropped to a minimum frequency of the VCO 15, it is necessary to return the driving frequency to a frequency region which is higher than the resonance frequency of the piezoelectric transformer 1. An operation for this purpose will be described in the following:
If it is not possible to supply the predetermined AC current IO (mArms) (or AC voltage VO (Vrms)) to the load 2, the output of the comparator 12 remains at the high level, which causes the driving frequency to continue to drop. If the output voltage of the integrating circuit 13 becomes lower than a minimum reference voltage Vmin which is set to correspond to the minimum frequency of the VCO 15, the comparator 14 outputs a reset signal of a high level to the integrating circuit 13. In response to this reset signal, the output voltage of the integrating circuit 13 is brought to a maximum voltage, which corresponds to a maximum frequency of the VCO 15. As a result, the driving frequency becomes the maximum frequency of the VCO 15, and the above mentioned operation will be repeated until the predetermined AC current IO (mArms) (or AC voltage VO (Vrms)) is supplied to the load 2.
Thus, when the input DC voltage VDD recovers the rated voltage or more, or when the cold-cathode fluorescent lamp connected as the load 2 starts to discharge, it is possible to supply the predetermined electric power.
As mentioned above, when the input DC voltage VDD is not less than the rated voltage, since the constant AC current IO (mArms) (or AC voltage VO (Vrms)) is supplied to the load 2, it is possible to supply the constant AC current IO (mArms) (or AC voltage VO (Vrms)) to the load 2, regardless of variations of the ambient temperature, the power source voltage and the load.
Referrring to FIG. 3, there is shown a block diagram of a prior art example of the DC-to-DC converter using a piezoelectric transformer, which is disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-4-210733.
As shown in FIG. 3, a DC voltage supplied from a DC power source 30 is supplied to a primary electrode driving circuit 31, which generates an AC voltage for driving primary electrodes of a piezoelectric transformer 1. Thus, secondary electrodes of the piezoelectric transformer 1 output a stepped-up AC voltage to an output rectifying circuit 32, which in turn supplies a rectified DC voltage to a load 2. Furthermore, the rectified DC voltage is also supplied to a detecting and amplifying circuit 35, which in turn supplies a detected and amplified signal to a variable frequency oscillator 33 so as to adjust an oscillation frequency of the variable frequency oscillator 33 supplied to the primary electrode driving circuit 31. Thus, by adjusting the driving frequency of the piezoelectric transformer 1, the step-up ratio of the piezoelectric transformer 1 is adjusted. In the above mentioned feedback loop, the DC voltage supplied to the load is stabilized.
The primary electrode driving circuit 31 shown in FIG. 3 adopts a resonance type converter configured to carry out a zero-voltage switching (ZVS) of the drive voltage or a zero-current switching (ZCS) of the drive current. Therefore, in order to adjust a duty ratio (duty factor) of switching means (not shown) included in the primary electrode driving circuit 31, thereby adjusting the switching timing of the primary electrode driving circuit 31, the rectified DC voltage is further supplied to a voltage-to-duty converting circuit 34, which in turn controls an on-off timing of the switching means (not shown) included in the primary electrode driving circuit 31, so that the primary electrode driving circuit 31 drives the piezoelectric transformer 1 with an adjusted half-wave sine wave.
Referring to FIG. 4, there is shown a block diagram of a prior art example of a DC-to-AC inverter using a piezoelectric transformer, which has a construction similar to that of the circuit shown in FIG. 3 and which is disclosed in "NIKKEI ELECTRONICS", No.621, pp147-157, Nov. 7, 1994.
In the inverter shown in FIG. 4, a forward type converter is constituted of an electromagnetic transformer T1 and a transistor Q1, to control a duty ratio and to realize the zero current switching (ZCS), so that a half-wave sine wave is generated to drive a piezoelectric transformer 1. An AC voltage stepped up by the piezoelectric transformer 1 is suppled to a cold-cathode fluorescent lamp constituting a load 2. A current flowing through the load 2 is converted by a resistor R10 and a diode D10 into a half-wave sinusoidal voltage, which is in turn supplied to a control IC (integrated circuit) 40.
In the control IC 40, the half-wave sinusoidal voltage is supplied through a buffer 45 to an integrating circuit 46, in which the half-wave sinusoidal voltage is converted into a DC voltage, which is in turn supplied to a VCO 42. In accordance with the magnitude of the receiving DC voltage, the VCO 42 generates a driving frequency for the piezoelectric transformer 1, and the driving frequency is supplied to a drive circuit 43 for driving the transistor Q1. As shown in FIG. 4, the control IC 40 additionally includes a starter circuit and load disconnection detection protection circuit 47, an input power source voltage detecting circuit 41, and an abnormal lighting detecting protection circuit and input voltage drop detecting protection circuit 44, which function to stop operation of the inverter shown in FIG. 4 when abnormality occurs in the power source voltage and/or in the load.
The piezoelectric transformer driving circuit shown in FIG. 1 is disadvantageous in the following points: For example, in the case that a cold-cathode fluorescent lamp is connected as the load for the piezoelectric transformer, for stabilizing luminescence of the cold-cathode fluorescent lamp, the circuit controls the driving frequency to increase or decrease the step-up ratio of the piezoelectric transformer for the purpose of controlling the current flowing through the cold-cathode fluorescent lamp to a predetermined desired value.
Generally, the piezoelectric transformer exhibits a maximum efficiency around a resonance frequency of the piezoelectric transformer, and the farther the driving frequency separates from the resonance frequency, the efficiency for transmitting an input power applied to the primary electrodes to an output power obtained from the secondary electrodes drops. Accordingly, if the piezoelectric transformer is caused to operate at a driving frequency exhibiting the maximum efficiency of the piezoelectric transformer, the driving frequency becomes the resonance frequency "fr" as shown in FIG. 5B, and then, the piezoelectric transformer operates with a maximum step-up ratio Av. Here, if the driving voltage of the piezoelectric transformer drops, the above mentioned current flowing through the cold-cathode fluorescent lamp cannot be maintained.
In the circuit shown in FIG. 1, a peak voltage VS1(Vo-p) of the driving voltage for the piezoelectric transformer is in proportion to a peak voltage Vd1(Vo-p) of the drain voltage of the transistor Q1, but as shown in FIG. 5A, since this peak drain voltage Vd1(Vo-p) of the transistor Q1 becomes about three times the input DC voltage VDD, the peak driving voltage VS1(Vo-p) is also in proportion to the input DC voltage VDD. Therefore, it is so set that, when the input DC voltage VDD is a minimum input voltage VDDmin(V), the driving frequency becomes the resonance frequency "fr" of the piezoelectric transformer in order to cause the piezoelectric transformer to operate with the maximum efficiency.
Under this condition, if the input DC voltage elevates, it is so controlled that the driving frequency for the piezoelectric transformer 1 is shifted or deviated to a frequency higher than the resonance frequency, with the result that the step-up ratio drops to supply a constant current to the load. For example, if the input DC voltage becomes two times the minimum input voltage, namely, 2VDDmin(V), the driving frequency is controlled to a frequency "fl" which makes the step-up ratio of the piezoelectric transformer 1 a half, and if the input DC voltage becomes 3VDDmin(V), the driving frequency is controlled to a frequency "f2" which makes the step-up ratio of the piezoelectric transformer 1 one third.
However, in the control method as mentioned above, when the power source voltage supplied to the driving circuit (boosting circuit) is high, it is not possible to avoid the piezoelectric transformer from operating at a driving frequency giving a low efficiency. Therefore, in the case that the piezoelectric transformer is operated in a wide power source voltage range, an averaged efficiency inevitably drops.
In this connection, the transistors Q1 and Q2 are required to have a high breakdown voltage so that these transistors are never broken down at a peak voltage occurring when the input DC voltage VDD is maximum. Accordingly, when the power source voltage range is wide, transistors having a high breakdown voltage are required. This results in an increased on-resistance which lowers the efficiency, and also increases the cost.
Another problem has been encountered in that, since the driving frequency is elevated to lower the step-up ratio of the piezoelectric transformer when the input DC voltage VDD becomes high, the driving waveform can no longer maintain a voltage resonance (series resonance) waveform, with the result that each of the transistors Q1 and Q2 is turned on before the voltage at the primary electrodes of the piezoelectric transformer becomes zero, and therefore, a zero voltage switching (ZVS) cannot be realized. Therefore, a large current flows through each of the transistors Q1 and Q2, so that the amount of heat generation becomes large, which is often destroys the transistor. Therefore, the widening of the input voltage range is inevitably restricted by a certain upper limit.
The above mentioned restrictions make it difficult to widen the input voltage range to greatly exceed beyond about two times the minimum input voltage. Accordingly, for example, the input DC voltage range becomes from 8 V to 16 V. To the contrary, a circuit utilizing an electromagnetic transformer can have a wide input voltage range of about three or four times the minimum input voltage (for example, from 5 V to 20 V in the DC voltage), since it can have a wide driving frequency range and further is not required to adopt the driving of the voltage resonance (series resonance) type or the current resonance (parallel resonance) type. Accordingly, the inverter and the DC-to-DC converter utilizing the piezoelectric transformer are narrow in the input voltage range, in comparison with those utilizing the electromagnetic transformer, and therefore, is restricted in the extent of application.
On the other hand, in the piezoelectric transformer driving circuits shown in FIGS. 3 and 4, since the resonance frequency range of the piezoelectric transformer is narrow, namely, since the Q factor of the piezoelectric transformer is high, the driving frequency can be adjusted only in the range of a few %. In addition, the on-off duty ratio of the transistor of the one-transistor type forward type converter can almost never be changed. As a result, there similarly occurs the phenomenon that the driving voltage for the piezoelectric transformer becomes large in proportion to the input voltage, and therefore, it is necessary to drive the piezoelectric transformer in a region of a low step-up ratio by shifting the driving frequency of the piezoelectric transformer from the resonance frequency of the piezoelectric transformer, completely similarly to the example shown in FIG. 1.
In the above mentioned examples, furthermore, it is required to so design that, when the input voltage is maximum, a magnetic saturation is never caused by a current flowing through a primary winding of the electromagnetic transformer for driving the piezoelectric transformer. Accordingly, the electromagnetic transformer is required to have a sufficient margin in capacity. Since the current flowing through the primary winding is in proportion to the input voltage, if the input voltage supplied to the driving circuit becomes double, the current flowing through the primary winding correspondingly becomes double. In this case, therefore, there is required an electromagnetic transformer allowing two times the current flowing when the circuit operates at a minimum voltage.
Conventionally, in order to cause a large current to flow through a winding of the electromagnetic transformer with no magnetic saturation, the size of the electromagnetic transformer inevitably becomes large. The piezoelectric transformer can be formed in a thinned shape in comparison with the electromagnetic transformer, but if the size of the electromagnetic transformer for driving the piezoelectric transformer becomes large, the advantage of the thin shape of the piezoelectric transformer can be exerted, so that a thinned shape inverter or power supply cannot be realized.
In the above mentioned piezoelectric transformer driving circuits, in order to enable the piezoelectric transformer driving circuit to operate in a wide input voltage range, the efficiency inevitably drops, or alternatively, the size of the device unavoidably becomes large. Therefore, the input voltage range cannot be satisfactorily widened.
From a different viewpoint, in the case that the piezoelectric transformer is used as a high voltage supply for lighting a backlight composed of a cold-cathode fluorescent lamp (cold-cathode tube), it is necessary to control the current flowing through the cold-cathode fluorescent lamp in order to adjust the backlight luminance. The cold-cathode fluorescent lamp has a characteristics of a negative resistance in which if the lamp current value becomes small, the impedance increases, and can be expressed by a resistance component RL and a capacitance component CL in an electric equivalent circuit, as shown in FIG. 2.
If an absolute value of the current flowing through the cold-cathode fluorescent lamp becomes small, a current flowing in a stray capacitance can no longer be ignored, so that there occurs a difference in current value between a high voltage side and a low voltage side of the cold-cathode fluorescent lamp, with the result that the luminance becomes uneven. This is inconvenient in the case that the backlight is constituted of the cold-cathode fluorescent lamp.
Under the above mentioned circumstance, in order to widely change the luminance, a so-called "burst light adjusting method" has been known, which is also called a "pulse width modulation light adjusting method". This method is characterized in that, the lamp current of the cold-cathode fluorescent lamp is intermittently turned on and off at predetermined intervals (for example, 210 Hz) which is not sensible to a human eye as a flicker, and a ratio of an on-time to an off-time is changed to equivalently change the luminance.
In order to realize this burst light adjusting method in the piezoelectric transformer driving circuit shown in FIG. 1, it is necessary to stop the alternating on-off operation of the two transistors Q1 and Q2 in order to intermittently stop the driving voltage applied to the piezoelectric transformer. This means that one side of the load inductance is opened, with the result that the accumulated current energy is discharged as a voltage energy, which generates a voltage surge far larger than the driving voltage of the piezoelectric transformer. Accordingly, it is required to provide a protection means for protecting the transistors from breakdown. This protection means may be constituted of a protection circuit incorporating therein a Zener diode.